Centrifuge principle

Centrifuge Principle

When trying to separate particles suspended in a liquid, one of the simplest methods is to let the mixture sit undisturbed for some time. In this process, gravity helps heavier particles sink while lighter ones float. For example, if we denote the volume of the particles as V, their density as ρ, and the density of the liquid as σ, the force acting on the particles due to gravity and buoyancy can be calculated as:

F = (ρ - σ) × V × g

Where g represents the acceleration due to gravity. If the particle's density ρ is greater than the liquid’s density σ, the net force F is downward, causing the particle to settle at the bottom. Conversely, if ρ is less than σ, the force is upward, making the particle float.

However, when the particles are very small or their density is very close to that of the liquid, this natural settling process can take an extremely long time, making it impractical for many applications. To speed up the separation, centrifuges are widely used in laboratories and industrial settings.

1-2-1 Centrifugation Principle

As shown in Figure 1-2-1, when particles in a solution move in a circular path around an axis, they experience centripetal acceleration. This motion creates a centripetal force, which is balanced by an apparent centrifugal force in the non-inertial reference frame of the rotating system. Although the centrifugal force does not exist in an inertial frame, it mimics the effects of a gravitational field in the rotating frame.

The centripetal force acting on a particle in a centrifuge is given by:

F = m × R × ω² = ρ × V × R × ω² (Equation 1-1)

Where:

• ρ – density of the particle
• V – volume of the particle
• R – radius of the circular path
• ω – angular velocity of rotation

The centrifugal force acting on the particle is equal in magnitude but opposite in direction to the centripetal force. Therefore, it can also be expressed using Equation 1-1. The strength of this artificial gravity depends on the particle's mass, the radius of rotation, and the rotational speed.

Because different particles have different densities and sizes, they will sediment at different rates depending on their position within the centrifuge. This allows for the efficient separation of various substances based on their physical properties. This principle forms the basis of centrifugation, and modern centrifuges are designed to optimize this process for scientific and medical applications.

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